1. Photospheric X-ray Spectroscopy of Neutron Stars Frits Paerels Columbia University, NY, and SRON National Institute for Space Research, The Netherlands Санкт-Петербург, 23-5-2008

2. Neutron Star Masses and Radii, 1932 - … “M = 1.5 M O, R = 10 km”

3. Mass Measurements From: Lattimer & Prakash, 2007, Physics Reports, 442, 109

4. Masses and Radii; Equation of State From: Lattimer & Prakash, 2007 Limits from fundamental considerations (c s < c, etc.); Spin rate; soon: moment of inertia from precession of S in binary

5. Attempts to use photospheric (X-ray) emission Example: photospheric emission from PSR B0656+14 (from Finley, Ögelman, Kızıloğlu, 1992, Ap. J. (Letters), 394, L21)

6. To measure radius: need distance ! At low resolution and/or S/N: cannot measure intrinsic spectral shape (everything looks like black body!) Data from TENMA (proportional counter) (from Waki et al., 1984, PASJ, 36, 819)

7. Potential of stellar photospheric spectroscopy atomic absorption: T eff, abundances, B, rotation; NS: gravitational redshift! X-ray burst spectrum: bright, probably low B, freshly accreting metals complements study of INS (high B, probably pure H or He)

8. Stark spectroscopy in Neutron Stars Pressure broadening should be observable! Hydrogenic ion (for simplicity): linear Stark effect: ΔE = 3a 0 |E|/Z (E perturbing electric field, Z nuclear charge, a 0 : Bohr radius dimensionally: ΔE ~ p  E ) so characteristic density: from integrating hydrostatic eq. down to τ T = 1:

9. The Stark width is not small! So for O VIII Ly  (853 eV): ΔE/E = 1/40! Higher order series members even more broadened: absorption spectrum strongly density-sensitive Stark broadening dominates over wide range of conditions: thermal Doppler: rotation: ΔE/E ≤ 1/40 for P rot ≥ 8 msec (R = 10 km) B-fields: ΔE Zeeman < ΔE Stark : ΔE Zeeman = μ B |B| < 20 eV: |B| < 3 x 10 9 G STARK WIDTH IS SENSITIVE TO DENSITY, HENCE TO g = GM/R 2 ; COMBINE WITH REDSHIFT (M/R): GET MASS AND RADIUS!

10. But will there be any bound electrons? Saha/Boltzmann for NS densities at high temperature:

11. High Resolution Photospheric Spectroscopy: the grating spectrometers on Chandra and XMM-Newton

12. Resolving powers: Chandra HETGS: 0.5-8 keV, E/ΔE up to ~ 1000 (Chandra LETGS:0.08-2 keV, E/ΔE up to ~ 2000) XMM-Newton RGS: 0.3-2 keV, E/ΔE up to ~ 700

13. PSR B0656+14/ Chandra LETGS (Marshall & Schulz, 2002, ApJ, 574, 377) RX J1856.5-3754/ Chandra LETGS (Drake et al., 2002, ApJ, 572, 996) nearly 500 ksec exposure! EW: λ/Δλ up to ~3500!

14. How about the X-ray Burst Spectrum ? LMXB EXO 0748-676 (Parmar et al. 1986, ApJ, 308, 199) P = 3.82 hr; dips; edge-on; frequent bursts EXO 0748-676/ EXOSAT ME 1985

15. Emission/absorption from accretion disk corona; He, H-like ions of N, O, Ne Direct disk emission blocked; ADC visible Disk shines through blocking material 30 X-ray bursts

16. Intermezzo: spectroscopy of the photoionized gas in the disk corona Gas is cool, photoionized: detect narrow RRC; OVII ‘triplet’: i + f > r i > f : collisional or radiative pumping to I : high density medium, or strong UV field from disk very unusual spectrum! He-like ion n=1,2

17. XMM/RGS: Cumulative spectrum of 30 X-ray bursts (Cottam, Paerels, & Mendez, 2002, Nature, 420, 51) If correct identification: gravitational redshift! z = 0.35

18. Irony: if z = 0.35 is correct, star is inside ISCO! accreting gas falls freely into atmosphere- resulting spallation may have observable effects (e.g. appearance of elements Z = 25, 24, 23, … )

19. Also: slow spin (fortuitous)! ν = 45 Hz, Villareal & Strohmayer, 2004, ApJ, 614, L121 Curve of growth for Fe XXVI n=2-3 (absorbing layer approximation)

20. Attempt verification: reobservation of the Fe XVII Balmer band with XMM/RGS (Cottam et al., 2008, ApJ, 672, 504)

21. Compare to 2000 observation

22. Attempt verification: search the Fe XVII Lyman band 37 X-ray bursts with Chandra HETGS; Paerels et al., in preparation.

25. Need spectroscopically accurate models for the spectrum Improvements needed: Using TLUSTY NLTE model atmospheres code (Ivan Hubeny, Thierry Lanz) NLTE important: Fe XXVI Lyα: Need complete Fe atomic structure model; consider all transitions Need model for Stark effect, pressure ionization: plasma-ion interaction, probably very nonlinear (not a dilute plasma) Want to model Lyman, Balmer, and Paschen (spectra exist!)